Could it be that the 9 speed does not work with 8 speed?
No.
How's the shifting? If shifting became less reliable at the same time the noise began, then you might have bent your hanger as CACycling stated.
High mileage chains can get a bit noisy as the rollers and plates wear, though noise may be lessened with fresh lube. Of course, you replaced the chain and still have the noise, so....
Have you checked the pulley with the chain removed? Perhaps a bearing is the culprit.